In the related art, in a vehicle such as a four-wheel vehicle, a rotatable connector device (SRC: Steering Roll Connector) for supplying power to an airbag device and other devices is mounted in a coupling portion between a steering wheel for steering and a steering shaft. The rotatable connector device includes a stator, a rotator that is assembled to the stator in a freely rotatable manner, and a flexible flat cable (FFC) that is wound and housed in an annular internal space formed by the stator and the rotator, and an end portion of the FFC includes a connecting structure that electrically connects the FFC and an outside.
The FFC includes a plurality of conductors that are arranged in parallel, a pair of insulating films that are arranged so as to sandwich the plurality of conductors, and an adhesive layer provided between the pair of insulating films, and has a laminated structure formed by the above plurality of conductors, the pair of insulating films and the adhesive layer. The conductors are made of, for example, a tough pitch copper, an oxygen-free copper, or the like. In addition, the insulating films each include an adhesive layer that is made of a polyester-based, polyurethane-based, polyamide-based, or polystyrene-based resin, and by bonding the above pair of insulating films with the adhesive layers interposed therebetween and with the plurality of conductors sandwiched therebetween, the conductors are insulated from each other, or the conductors are insulated from an outside.
As the above conductors, for example, there is proposed a conductor for a flat cable that is made of a tough pitch copper or the like and in which occurrence of a crack is suppressed even when bending deformation is repeatedly applied with a bending radius of about 8 mm (Japanese Patent No. 5342712).
As another conductor, there is proposed a conductor for a flat cable made of a copper alloy to which one or more kinds of B, Sn, In, and Mg are added at 0.005% to 0.045% in total, and in which crystal grains are refined to 7 μm or smaller (Japanese Patent No. 3633302).
There is proposed a flat conductor obtained by performing heat treatment on a flat-shaped conductor that includes a base metal being a copper alloy that is an oxygen-free copper (99.999 wt % Cu) to which 0.3 wt % or less of Sn and 0.3 wt % or less of In or Mg is added, or being a copper alloy that is an oxygen-free copper (99.999 wt % Cu) to which 10 wt % or less of Ag is added and that includes a surface on which Sn is plated, the flat conductor having a tensile strength of 350 MPa or higher, an elongation of 5% or more, and an electrical conductivity of 70% IACS or higher (Japanese Patent No. 4734695).
In the technique of Japanese Patent No. 5342712, however, in order to maintain a bending property of the conductor, the bending radius needs to be about 8 mm, so that reduction of a rotatable connector device in size cannot be achieved, and on the other hand, in a case where the rotatable connector device is reduced in size, sufficient elasticity and bending property cannot be obtained. In particular, in a flat cable using a conductor made of a tough pitch copper of the related art, a sufficient elasticity is not obtained by reduction in size, and it is thus difficult to wind up or rewind the flat cable. In addition, because the bending property is poor, a lifetime required for the rotatable connector device is not achieved, and signal transmission by the conductor cannot be performed sufficiently.
In addition, in the technique of Japanese Patent No. 3633302, by merely performing grain size control by defining the kind and content of additional elements in the copper alloy, a bending property of the conductor is insufficient.
Moreover, in the technique of Japanese Patent No. 4734695, although it is described that an elongation of 5% or greater is essential, and when the elongation is out of the range, the rigidity is high, which makes it difficult to fold or bend the conductor and may cause the conductor to buckle when folding or bending the conductor, it is revealed that a bending property of the conductor is insufficient even when the elongation is 5% or greater. In particular, in recent years, with the development of automobiles having higher performance and higher functions, improvement in durabilities of various devices installed in an automobile is required from the viewpoint of improvement of reliability, safety, and the like, and further improvement of the bending property of the flat cable in a rotatable connector device is required.